In the practice of logging-while-drilling (LWD), measurement-while-drilling (MWD) and wireline logging, it is well known that by studying the propagation characteristics of an electromagnetic wave, useful clues regarding the characteristics of the earth formations can be derived. To probe sections of the earth surrounding the borehole, a pair of transmitters can be positioned within a well borehole to radiate an electromagnetic field at a particular interrogation frequency. This electromagnetic wave is influenced by the electromagnetic energy shed back from the formation. A pair of spaced differential loop antenna receivers is conventionally positioned within the borehole to measure, for example, the attenuation and/or phase shift of the electromagnetic wave as it passes between each receiver loop antenna. Various methods for analyzing the measurements to derive estimates of certain characteristics of the earth formations surrounding the well borehole are well known.
It is of increasing importance in oil and gas exploration to obtain accurate and reliable measurements of an electromagnetic wave investigating a formation. However, the accuracy of the information derived from the measurements can be degraded by the effects of magnetic field mutual cross-coupling between receiving loop antennae. Receiver cross-coupling typically results from significant circulating alternating current that is induced in a receiver loop antenna in response to an electromagnetic wave. This alternating current tends to produce a secondary electromagnetic field that can have a corrupting influence on the primary electromagnetic wave generated by a transmitter. The secondary electromagnetic field will affect the measurements obtained by any other receiving loop antenna in close proximity to the first receiver, producing an error component due to the cross-coupling. The receiving antenna closest to an active transmitter tends to receive a stronger signal and produce greater circulating currents than subsequently spaced receiving antennae. Accordingly, the magnitude of the undesirable secondary electromagnetic field radiated by an antenna tends to be greater from the receiving antenna that is closer to an active transmitter, and the magnitude of the error component due to cross-coupling tends to be higher in the next subsequently spaced receiving antenna.
Considerable effort has been expended by the industry to compensate measurements for the cross-coupling error. For example, one known method includes a calibration procedure where, under laboratory conditions, the cross-coupling error for each frequency of interest is measured and stored. Thereafter, each subsequently measured value is adjusted accordingly. Such methods tend to be cumbersome, may introduce new sources of error and may create maintenance restrictions. For example, extra components may be needed on the receiver circuits to simulate the cross-coupling effect in the lab. Parameters such as the distance between receivers, which will vary with temperature, are critical to the accuracy of the estimate of the cross-coupling error. In addition, the calibration may be invalidated by the replacement, in a receiver antenna system, of a failed component that influences the cross-coupling.
Eliminating the source of the cross-coupling error in downhole tools has proven to be problematic in the industry. For example, methods commonly employed to counter the excessive signal loss resulting from a lengthy coaxial cable, typically having a characteristic impedance less than 100 ohms, involve matching the receiver loop antenna impedance to the impedance of its load. However, the matching of receiver loop antenna impedance to that of its load typically results in significant circulating currents being induced in receiver loop that create receiver cross-coupling. Other methods employed by prior art systems include spacing receiver antennae as far apart as possible to reduce the effects of cross-coupling, such as locating each of a pair of receiver antennae on opposing sides of a pair of transmitters.
A strong pick up signal is an important consideration in obtaining accurate measurements. Prior art downhole tools that match the receiver antenna impedance to a load comprising a lengthy coaxial cable tend to employ single turn antennae, even though a multiple turn loop antenna typically provides the advantages of a strong pickup as compared to having a single turn antenna. A multiple turn loop antenna, in the range of 6 to 12 inches diameter, commonly exhibits several hundred ohms of impedance at 2 Mhz. Thus, prior art methods for matching the receiver antenna to the load impedance combined with the use of a step-up transformer, tend to limit a receiver loop antenna to no more than a single turn.
A further limitation of prior art receiver loop antenna systems is their inability to be simultaneously series tuned at multiple interrogation frequencies. It well known that it is advantageous to utilize multiple interrogation frequencies to probe earth formations with electromagnetic waves. Certain attributes of the earth formation are discoverable only when the interrogation frequency is of a specific range. Lower frequencies are able to investigate deeper regions of the earth for a given transmitter and receiver spacing. Also, lower frequencies often mitigate borehole effects. Higher frequencies yield higher phase shift and attenuation values for a given resistivity, which is advantageous for increased accuracy in highly resistive formations of commercial interest. In LWD and MWD systems where measurements are commonly obtained while the measuring tool is rotating and moving axially through the borehole, greater and more accurate information about the surrounding earth formation can be derived by obtaining simultaneous measurements of a plurality of interrogation frequencies.
There is therefore a need in the art for receiver systems, deployable within a borehole, that can utilize a loop antenna having multiple turns, as opposed to a single turn, to enable the antenna to pick-up a strong signal from which a more accurate measurement of particular attributes of an electromagnetic wave can be derived. Also, there is a need for a loop antenna that can be simultaneously series tuned at plurality of interrogation frequencies to enable it to simultaneously and accurately pick-up the plurality of interrogation frequency components from an electromagnetic wave. In addition, there is a need for a method for decreasing the design, manufacture, and maintenance cost of systems, that deploy a pair of loop antenna receivers downhole, while still diminishing the undesirable effects of mutual cross-coupling. Furthermore, there is an ever present need for downhole antenna systems that are stable over a wide range of temperatures, and that provide increasingly accurate and greater amounts of information about the earth formations surrounding a borehole.